This invention relates to internal combustion engines, and more particularly it is concerned with a gas sealing device for providing a gas seal between the cylinder and the piston of an internal combustion engine.
An internal combustion engine is known which comprises a cylinder, a piston arranged in the cylinder for reciprocatory movement, and a gas sealing device for providing a gas seal between the piston and the inner surface of the cylinder, wherein a combustion chamber is defined between the cylinder and the top surface of the piston and wherein the gas sealing device includes a piston ring fitted in a piston ring groove formed on the outer circumferential surface of the piston. The gas sealing device used in this type of internal combustion engine has hitherto been available in a variety of known forms. Gas sealing devices of the prior art have had the disadvantages that the portion of the piston disposed above the top piston ring groove for receiving the piston ring therein is reduced in strength, that no positive sealing function is obtainable, that seizing occurs at the interface between the piston ring and the piston ring groove, and that the power of the internal combustion engine is reduced.
FIGS. 1-3 show examples of internal combustion engines provided with gas sealing devices of the type described of the prior art. In order that the aforesaid disadvantages of the prior art may be fully understood, the gas sealing devices shown in FIGS. 1-3 will be described hereinafter.
FIG. 1 shows an internal combustion engine comprising a cylinder 1, a piston 2 arranged in the cylinder 1 for reciprocatory movement vertically in the figure, and a gas sealing device for providing a gas seal between the piston 2 and an inner surface 7 of the cylinder 1. The cylinder 1 and the piston 2 define therebetween a combustion chamber 3, and the gas sealing device includes a piston ring 4 fitted in a piston ring groove 5 formed on a circumferential surface of the piston 2. The piston ring 4 shown in FIG. 1 is substantially in the form of a letter L in cross-sectional shape and includes an outer circumferential surface 6 maintained in pressing contact with the inner surface 7 of the cylinder 1. The piston ring groove 5 opens in the combustion chamber 3 and consequently a top surface 8 of the L-shaped piston ring 4 is exposed in the combustion chamber 3. A gap 9 is formed between surfaces of the piston ring groove 5 and the piston ring 4. When the internal combustion engine is in operation, the pressure of gas from the combustion chamber 3 applied to inner circumferential surfaces 10 and 10' of the piston ring 4 urges the piston ring 4 to move radially thereof, to thereby force the outer circumferential surface 6 of the piston ring 4 against the inner surface 7 of the cylinder 1. If the pressure of gas is high, then the piston ring 4 is forced against the inner surface 7 of the cylinder 1 with a relatively high force; and if the pressure of gas is low, then the piston ring 4 is forced against the inner surface of the cylinder 1 with a relatively low force. That is, when the pressure of gas is high and consequently a high sealing ability is required, the piston ring 4 is pressed against the inner surface 7 of the cylinder 1 with a high force due to the high gas pressure, to provide a desired high degree of seal. The gas sealing device shown in FIG. 1 is capable of controlling the force with which the piston ring 4 is forced against the inner surface 7 of the cylinder 1 in accordance with the pressure of gas in the combustion chamber 3, so that the force with which the piston ring 4 presses against the inner surface 7 of the cylinder 1 can be reduced when the internal combustion engine is inoperative, to thereby minimize a friction loss between the piston ring 4 and the inner surface 7 of the cylinder 1. By virtue of this feature, the constructional form shown in FIG. 1 has particular utility in application to a two-cycle engine of high speed and high power.
However, the internal combustion engine shown in FIG. 1 has the disadvantage that with the top surface 8 of the piston ring 4 exposed in the combustion chamber 3, a peripheral portion 11 of the piston 2 overlying the piston ring groove 5 has a small thickness t extending axially of the piston 2 and consequently the strength of the peripheral portion 11 is reduced. A portion of the peripheral portion 11 disposed on the exhaust port (not shown) side is exposed to high temperatures and liable to be damaged.
To cope with this situation, a proposal has been made to form the piston ring groove 5a in a position remote from a top surface 12a of the piston 2a to provide a top land 13a between the top surface 12a and piston ring groove 5a as shown in FIG. 2, so as to increase the thickness of the peripheral portion 11a of the piston 2 overlying the piston ring groove 5a. By this arrangement, the thickness of the peripheral portion 11a of the piston 2a is increased and the strength of the peripheral portion 11a is increased. However, in this construction, the pressure of gas produced in the combustion chamber 3a does not act readily on the inner circumferential surfaces 10a and 10a' of the piston ring 4a of the L-shape, with a result that the gas sealing action relying on the pressure of gas of combustion characteristic of an L-shaped piston ring is prevented from achieving satisfactory results and the power of the engine tends to show a reduction. Also, with a reduction in the pressure of gas acting on the L-shaped piston ring 4a, the radial movement of the L-shaped piston ring 4a is reduced in intensity. Combined with a reduced radial movement of the piston ring 4a, a reduction in the temperature in the piston ring groove 5a below the temperature in the piston ring groove 5 shown in FIG. 1 increases the possibility of occurrence of seizing in the interface between the piston ring 4a and the piston ring groove 5a due to the deposition of carbon in the annular groove 5a.
To avoid the occurrence of seizing in the interface between the piston ring 4a and the piston ring groove 5a, a proposal has been made to form, as shown in FIG. 3, a keystone piston ring groove 5b on the outer circumferential surface of the piston 2b in a position corresponding to that of the piston ring groove 5a shown in FIG. 2 and to fit an ordinary keystone ring 4b in the keystone piston ring groove 5b, so as to provide a gas seal to the cylinder 1b and piston 2b. In this construction, it is necessary to increase the force urging the piston ring 4b against the cylinder 1b when the engine is inoperative, so that the friction between the outer circumferential surface 6b of the piston ring 4b and the inner surface 7b of the cylinder 1 is increased and the power of the engine is reduced.